The invention relates to a computer method for generating a display picture representing a set of object elements including a brush object element. U.S. Pat. No. 4,631,690 to the same assignee, herein incorporated by reference, discloses a multiprocessor system for forming a color picture from object elements defined in a hierarchical data structure. The present invention is not restricted to a multilevel hierarchy. The present invention may be applied to a two-color system (e.g. black and white) and to a one-processor implementation. The state of the art shows that even for a complicated pictorial situation the number of calculation steps necessary for color-assignment to a particular pixel remains limited to a low average value. For the specific case of Bezier curves and shapes the calculation generally boils down to elementary shift and add operations that straightforwardly lead to a silicon implementation of the processing. The nature of the present approach is point-driven versus the conventional object-driven approach now generally in use. The present invention envisages the use of brushes. For conceptualization thereof, a Bezier curve is defined as a primitive element that is expressed by a Bezier formula. A path is a concatenation of curves; a path may be closed or non-closed. A path may consist of a single curve. A shape is described by one or more closed paths. A certain inside versus outside definition specifies the points of the shape.
For better explanation of the description to follow, the following three frames of references are specified:
first there are continuous or analog application coordinates in which the objects to be displayed are conceptually modelled; these will not be considered hereinafter;
second there are discrete application coordinates, which are used for computer models of these objects and for digital calculations to generate results that are to be displayed;
finally, discrete device coordinates specify pixels in the picture field to be displayed according to a particular color specification.